1. Field of the Invention
This invention relates to a novel additive for an aqueous drilling fluid and a method for drilling wells into subsurface formations utilizing drilling fluids containing said additive, and more particularly to an additive for reducing the fluid loss or filtration rate of aqueous drilling fluids which are saturated or essentially saturated with respect to calcium hydroxide and contain high levels of salinity.
2. Description of the Prior Art
Drilling fluids, or drilling muds as they are sometimes called, are generally slurries of clayey solids used in the drilling of wells in the earth, such as are drilled for the purpose of tapping subterranean deposits of petroleum, gas, and other fluid materials. Drilling fluids have a number of functions, the most important of which are removing formation cuttings from the well, sealing off permeable formations of gas, oil or water which may be encountered at various levels as the well is drilled into subterranean formations, cooling and lubricating the drilling tool and drill pipe which carries the tool, and holding the cuttings in suspension and the event of a shut-down and drilling and pumping of a drilling fluid.
An ideal drilling fluid is a thixotropic fluid, i.e., a fluid whose viscosity decreases as the degree of agitation or shear increases. The drilling of wells utilizing drilling fluids, agitation or shear is caused by pumping or otherwise circulating the fluid through the drill string. When agitation or shearing caused by circulation is halted, however, the fluid should gel or form a gel structure which will support the drill cuttings to prevent them from falling back into the bottom of the hole. The rate of gel formation must be such as to permit the cuttings to fall only a short distance before the gel structure is sufficiently strong to support them. It is important to maintain a degree of gelation and the rate of gelation within narrow limits, since excessive gel formation would be detrimental to resumption of drilling operations, and will also tend to entrain gas in the drilling fluid. Entrainment of gas in drilling fluids leads to a substantial reduction of a density or specific gravity of the drilling fluid, which is detrimental to continued safe drilling of the well in instances where high pressure formations are encountered. If the gelation rate and gel strength is below the desired range, formation cuttings and other solid materials such as weighting material will fall to the bottom of the hole which results in sticking of the drill pipe.
In many instances it is also highly desirable to control the rate of filtration, generally referred to in the art of oil well drilling fluids as "fluid loss". Many times wells are drilled into essentially impermeable formations, and when drilling through such formations the filtration rate or fluid loss of the drilling fluid is not essential and generally no attempt need be made to control it. When a well must be drilled through a permeable formation, however, some additive must be included in the drilling fluid composition to reduce the rate of filtration into the permeable formation. Excessively high filtration rates will result in the formation of a filter cake at the interface between the well bore and the formation, with the fluid phase of the drilling fluid passing into the formation and the suspended particulate phase accumulating in the filter cake at the well bore face. Filter cake accumulation rate is roughly parallel to gross filtration rate, and the thickness of the filter cake can become sufficiently large that interference with the drilling operation such as sticking of the drill pipe can result. For these and other reasons, it is frequently necessary to add a material to the drilling fluid in order to reduce the filtration rate or fluid loss to an acceptably low value. Fortunately, compositions are known which can reduce the fluid loss or filtration rate of conventional drilling fluid systems quite effectively. For example, prehydrolyzed starch and carboxymethylcellulose are both well known in the art of oil well drilling fluid technology for the purpose of reducing the filtration rate or fluid loss of aqueous based drilling fluids.
Special purpose drilling fluids are used in certain geographical areas where the well must penetrate formations known as heaving or sloughing shales. An aqueous drilling fluid containing excess calcium hydroxide and other water soluble salts having a prescribed filtrate alkalinity is described in U.S. Pat. No. 2,802,783 (1957) W. J. Weiss, et al, and this drilling fluid has been very successful for drilling certain types of "mud making" heaving shales encountered in the Gulf Coast region. A distinctly different aqueous drilling fluid for controlling a different type of heaving shale is disclosed in pending application, Ser. No. 309,327 filed Nov. 24, 1972, now abandoned which has been highly successful for drilling through Illitic, non-mud making heaving shales encountered in the Delaware Basin area of West Texas and New Mexico, specifically the Pennsylvanian-Mississippian interval. This latter drilling fluid, a low solid shale control drilling fluid, is also saturated with respect to calcium hydroxide and has excess or undissolved calcium hydroxide dispersed therein, and further contains a small amount of another calcium salt having a solubility greater than the solubility of calcium hydroxide, and which may additionally contain appreciable quantities of sodium chloride, or which may even be saturated with respect to sodium chloride. This novel low solid shale control drilling fluid is distinct from the first mentioned shale control drilling fluid in that it is a highly flocculated system, being characterized by an API fluid loss in excess of 100 cc's and a ratio of yield point to plastic viscosity less than 1. Most drilling fluids, including the first described shale control drilling fluid, are responsive to treatment with conventional fluid loss or filtration rate controlling additives such as prehydrolyzed starch or carboxymethylcellulose. Starch may be used effectively in any drilling fluid containing appreciable quantities of soluble calcium or containing a high level of salinity. If the system contains both high salinity level and high soluble calcium ion concentration, neither starch or carboxymethylcellulose are effective for controlling the fluid loss of the drilling fluid. Although the low solid shale control drilling fluid may frequently be used during a prolonged drilling period with essentially no additive present for the purpose of controlling fluid loss, there are instances where it is desirable to use such a drilling fluid for penetrating an Illitic heaving shale interval, in the drilling of a well in an area where known permeable formations will be encountered either above or below the Illitic heaving shale interval. Accordingly, there is a substantial commercial need for a method for reducing the fluid loss of a low solid shale control drilling fluid having both high salinity and high soluble calcium levels, especially a drilling fluid which is saturated with respect to calcium hydroxide and essentially saturated with respect to sodium chloride, which additive will not otherwise distort either the rheological or chemical properties of the drilling fluid.